Because calcium phosphate compounds are easily absorbed and fixed to human tissues, they are neither rejected by living bodies nor cause necrosis. Accordingly, they are commercially available for artificial bones, artificial teeth, fillers (hereinafter referred to as “bone fillers”) in various forms such as sintered bodies, granules, putties, etc.
Most calcium phosphate products are hydroxyapatite ceramics having a chemical formula of Ca10(PO4)6(OH)2. The hydroxyapatite ceramics have sufficient strength as bone fillers, accelerating bone regeneration in bone-lost portions. The bone filler of hydroxyapatite embedded in a living body is attached and/or fixed to a living hard tissue. The capability of accelerating bone regeneration in bone-lost portions is called “bone conduction.”
Because the embedded bone filler is not fully fixed to a living bone until it is attached and fixed thereto, troubles are likely to occur. Though hydroxyapatite bone fillers have bone conduction, they are not quickly adhered or fixed to a living bone. The attachment or fixing of the hydroxyapatite bone filler to a living bone usually takes 4-5 weeks. Recently, research is vigorously conducted to shorten a time period for attaching and/or fixing the bone filler to a living hard tissue.
As an index of the capability of forming bone in a living body, there is “bone-inducing capability,” in addition to the bone conduction. The bone-inducing capability is the capability of forming bone in other portions than bone-lost portions. When a material having the bone-inducing capability is used for a bone filler, the formation of bone is induced even in a gap between the bone filler and a living bone, effective to shorten a time period for the bone filler to be fixed to a living hard tissue.
Tricalcium phosphate (TCP) soluble in a living body is known as a bone-inducing material. Because TCP is more soluble in a living body than hydroxyapatite, it is expected that the TCP embedded in a living body makes the differentiation, induction, etc. of osteoblasts easy. However, a filler made only of TCP cannot exist stably in a living body because it is extremely soluble. The TCP filler is likely to have insufficient strength after a long period of time passes from embedding. Accordingly, it cannot be used as a bone filler for a portion needing mechanical strength. Development has thus been conducted to provide a filler formed by a composite comprising hydroxyapatite stable in a living body and high-solubility TCP.
A bone filler of hydroxyapatite and TCP can be produced, for instance, by arranging TCP powder around a sintered body of hydroxyapatite powder and sintering the resultant composite. The hydroxyapatite powder and the TCP powder used for this method can be produced, for instance, by mixing an aqueous phosphoric acid solution with an aqueous calcium solution. However, which is formed among many crystal systems of calcium phosphate depends on the ratio of phosphoric acid to calcium, the pH of a mixed solution, etc. Accordingly, production conditions change as a synthesis reaction proceeds, failing to obtain homogeneous hydroxyapatite or TCP.
JP6-237984 A discloses an implant material for a living body, which is a calcium phosphate sintered body comprising a hydroxyapatite phase and a tricalcium phosphate phase, with different constituent proportions between an inner crystal layer and an outer crystal layer. It also disclose as methods for producing this implant material, (a) a method of coating or immersing a porous body of calcium phosphate having a calcium/phosphorus (Ca/P) atomic ratio of 1.4-1.75 with or in a calcium phosphate frit or its mixture with hydroxyapatite powder, and sintering it, and (b) a method of coating or immersing the above porous body of calcium phosphate with or in a phosphorus compound or phosphoric acid, and sintering it. These methods can produce implant materials for a living body, whose crystal phases are different in constituent proportions between inner layers and outer layers, the percentage of TCP in the outer layers being relatively high. However, it is difficult to surely form the outer layer with TCP, and to control the thickness of the outer layer, resulting in the production of limited implant materials.